1. Field of the Invention
One disclosed aspect of the embodiments relates to photoelectric conversion apparatuses and image pickup systems and, more particularly, to a photoelectric conversion apparatus including analog-to-digital (AD) converters and an image pickup system including AD converters.
2. Description of the Related Art
Image pickup apparatuses including AD converters are known. Japanese Patent Laid-Open No. 2010-45789 describes an AD converter that compares an analog signal with a reference signal that changes with respect to time. Specifically, Japanese Patent Laid-Open No. 2010-45789 describes that a digital signal with a resolution corresponding to a signal level of a pixel signal output from each pixel is obtained by changing a slope of the reference signal in accordance with the signal level of the pixel signal.
In Japanese Patent Laid-Open No. 2010-45789, variations in an offset voltage of a comparator included in an AD converter is not taken into account. Thus, AD conversion may not be performed precisely.